1. Field of the Invention
The present invention relates to an electroluminescent device (EL device) using an organic film containing an organic coloring material.
2. Description of the Related Art
In recent years, the research and development of organic EL devices used as display and illumination devices have flourished. For example, Shogo Saito, the Kyushu University, reported an EL device having an organic two-layered structure using metal electrode/aromatic coloring material/polythiophene/metal electrode (J.J. Appl. Phys., 25, L773, 1986). In this device, however, the thickness of the organic film is 1 .mu.m or more, and a voltage applied to the device is as high as 100 V or more. To the contrary, C. W. Tang et al., Eastman Kodak, reported an EL device having an organic two-layered structure of Mg-Ag/Alq.sub.3 /diamine/ITO (Appl. Phys. Lett., 51, 913, 1987). According to this report, the thickness of the organic film is set to be 100 nm or less to drive the device at a voltage of 10 V or less, thereby obtaining a device having a practically sufficient luminance. The above EL devices have the following characteristic features. An electron injection coloring material is combined with a hole injection coloring material to obtain an organic two-layered structure. The thickness of the organic film is minimized, a metal electrode having a low work function is used for electron injection, and an organic material formed by vacuum deposition or sublimation is selected to prevent electrical defects in fabrication of the EL device. In addition, Shogo Saito, the Kyushu University, proposed a device having an organic three-layered structure of electron injection layer/light-emitting layer/hole injection layer. In this structure, high-luminance emission can be obtained by employing an organic material having a high photoluminescence as a light-emitting layer (J.J. Appl. Phys., 27, L269, 1988).
C. Adachi, T. Tsutsui, and S. Saito (Appl. Phys. Lett., 56, 799 (1990)) proposed a device having an organic three-layered structure of Mg.multidot.Ag/electron transport film/light-emitting film/hole transport film/ITO. According to this report, when phenylbenzoxadiazole (PBD), dimethyltetraphenylbenzidine (TAD), and tetraphenylbutadiene molecules were respectively used for the electron transport film, the hole transport film, and the light-emitting film. At a drive voltage of 10 V and a current density of 100 mA/cm.sup.2, emission having a luminance of 700 cd/m.sup.2 was obtained. However, when another light-emitting film with lower photoluminescent efficiency is used, the luminance is greatly decreased and the emission wavelength is sometimes shifted to a longer wavelengthes.
Regarding the luminance required for a practical usage, organic EL devices have almost been satisfied. However, many technical problems are left unsolved in luminous efficiency, device life time, and device fabrication process. The luminous efficacy defined as the ratio of emission photo densities to electric charge carriers is a maximum of 1% at present and is generally about 0.1%. A low luminous efficiency indicates that a current which does not contribute to emission flows across the electrodes. This current causes a decrease in device life time because the Joule heat generated deforms the layered device structure and sometimes decomposes the organic layers. Therefore, in order to obtain a practical organic EL device, demand has arisen for increasing the luminous efficiency from at least several % to about 10%.
In order to increase the luminous efficiency of the organic EL device, the device structure and the electrical characteristics of materials used must be optimized. However, only the qualitative characterization for them are available at present. Electron (or hole) transportability, electron (or hole) injection properties, electron donor properties, electron acceptor properties, and emission properties of the oranic films are key factors affecting the device performances. Therefore, the optimal conditions for designing the device structure are not yet sufficiently established.
In order to arrange a full color display using organic EL devices, three-colored EL devices of red, green, and blue (RGB) are required. As is well known, it is difficult to achieve blue emission with a wavelength shorter than 470 nm (2.6 eV). This is partly because the wide optical band gap is susceptible to influences of impurities in the EL device. For this reason, even if blue emission is obtained, its luminous efficiency and luminance are low. In addition, unexpected long-wavelength emission which may be caused by indirect transition from an impurity level is observed, and its color purity is not so high. This problem will be described in detail below.
According to the above definitions, the conventional organic EL device has a basic structure in which an anode (second electrode), a second organic film consisting of hole injection molecules, a first organic film consisting of electron injection molecules, and a cathode (first electrode) are sequentially stacked. In this structure, assume a device for realizing blue emission (2.6 eV). It is known that the maximum energy of emission of an organic material is shifted to a low energy from the band gap (absorption end) by about 0.5 eV (Stokes shift). When this is taken into consideration, a material having a band gap of 3.0 eV or more is preferably used to obtain blue emission.
An electrode having as a large work function as possible is used for the second electrode. An ITO transparent electrode has a work function of 4.8 eV. An electrode having as a small work function as possible to facilitate electron injection is used for the first electrode. For example, an Al electrode has a work function of 4.2 eV.
In a conventional device, a material which is easily subjected to electron injection from the first electrode (Al) is used as the first organic film. Thus, the material having a high electron affinity in respect of an energy level, i.e., a conduction band level of 3.8 eV or more is selected. On the other hand, a material which is easily subjected to hole injection from the second electrode (ITO) is used as the second organic film. Thus, the material having a low ionization potential in respect of an energy level, i.e., a valence band level of 5.4 eV or less is selected. When the band gap of an organic film is 3 eV, the conduction band level of the second organic film is 2.4 eV or less, and the valence band level of the first organic film is 6.8 eV or more.
A junction of this device in a flat band state is shown in FIG. 1. In this device, the junction barrier of an interface of two organic films has a value exceeding 1.0 eV on both the conduction and valence band sides. In this case, when stronger donor molecules which tend to be injected with holes and stronger acceptor molecules which tend to be injected with electrons are used, i.e., when molecules having a low ionization potential and molecules having a high electron affinity are used, the junction barrier is increased.
In such a device, when a positive bias is applied to the second electrode, an operation shown in FIGS. 2 and 3 is ideally performed. More specifically, electrons are injected from the first electrode to the first organic film and are stored at the interface between the first and second organic films. Meanwhile, holes are injected from the second electrode to the second organic film and are stored at an interface of the second and first organic films. The electrons and the holes form an electrical double layer through the interface of the organic films. A strong electric field is induced in the electrical double layer, so that the electrons or positive holes tend to be tunnel-injected. As a result, the electrons are tunnel-injected to the second organic film and are recombined with the positive holes in the second organic film, thereby emitting light. Meanwhile, the positive holes are tunnel-injected into the first organic film and are recombined with the electrons in the first organic film, thereby emitting light. If the device is assumed to be operated on the basis of this ideal principle, an emission wavelength is determined by the band gaps of the first and second organic films. Even if the Stokes shift is taken into consideration, emission in the blue range having the central energy of about 2.7 eV is assumed to be obtained.
However, in the device manufactured by the state-of-the-art process and having the junction in FIG. 1, electroluminescence occurs in the red region of 2.0 eV or less because a structural disturbance is found to occur at the interface of the organic films in the device manufactured by the state-of-the-art process. This will be described below.
Assume that a first organic film is deposited on a second organic film. In a deposition apparatus, molecules constituting the first organic film are bombarded against the surface of the second organic film at high speed. As a result, as shown in FIG. 4, the molecules constituting the first organic film are diffused in the second organic film. The diffusion depth may reach several 10 nm. As shown in FIG. 5, the structure of the second organic film is disturbed upon bombardment of the electrons thereagainst, and an intermediate layer, in which the molecules constituting the second organic film are mixed with those constituting the first organic film at almost the same concentration, may be formed. As shown in FIG. 6, when these phenomena occur, a new level (to be referred to as a charge transfer (CT) level hereinafter) caused by the conduction band level of the molecules constituting the first organic film is formed in the second organic film. At this time, the electrons injected from the first electrode to the first organic film are not stored at the interface between the organic films but are injected in the second organic film through the CT level, and are recombined with the holes in the valence band of the second organic film. More specifically, assume that the rate of electrons injected through the interface of the organic films is defined as k.sub.inj, and that a recombination rate caused by transition from the CT level is defined as k.sub.CT. When the junction barrier has a value exceeding 0.6 eV as in the conventional device, an injection probability caused by the thermal excitation process is greatly decreased to make k.sub.inj &lt;&lt;k.sub.CT.
Since the recombination energy is about 2.0 eV or less, only long-wavelength emission is obtained in this device. In addition, when stronger donor molecules and stronger acceptor molecules are used, the position of the CT level comes close to the valence band of the second organic film, so that emission is shifted to a longer wavelength. As a result, the blue emission EL device described above cannot be realized. In addition, since a CT emission spectrum is generally broad so as to have a half value width of 0.4 eV, and a color mixture with emission caused by recombination of the tunnel-injected electrons and positive holes also occurs, the color purity is greatly decreased. In order to prevent this CT emission, it is preferable to prevent diffusion of molecules during stacking of the organic films. It is, however, very difficult to realize this in accordance with the state-of-the-art process.
Even in the absence of diffusion of molecules during stacking of organic films, when the conduction band of the first organic film is close to the valence band of the second organic film, the electrons and holes stored at the junction interface of the first and second organic films may be indirectly recombined through the junction interface. At this time, the emission wavelength corresponds to an energy difference between the conduction band of the first organic film and the valence band of the second organic film. In this case, long-wavelength emission occurs, and emission having a wavelength distribution determined by the band gaps of the first and second organic films cannot be observed.
As described above, in the conventional organic EL devices, electrical junction conditions and structural conditions of the thin films at the interface are influenced by a variety of factors, it is difficult to manufacture an organic EL device which can be ideally operated. That is, in the conventional organic EL devices, since emission based on the above-mentioned indirect recombination occurs in addition to the emission having a wavelength determined by the band gaps of the first and second organic films, the color purity and hence the luminous efficiency are decreased accordingly.